1. Field of the Invention
The present invention relates to a method to driver a semiconductor laser diode
2. Related Background Art
Various prior arts have disclosed an algorithm and a circuit to drive a semiconductor laser diode so as to make the optical output power and the extinction ratio (hereafter denoted as ER) invariant with respect to temperatures and the ageing. For instance, a Japanese patent application published as JP-H11-135871A has reported one of such algorithms and circuits where the LD is driven by superposing a bias current slightly greater than the threshold current of the LD with a pulse current, which is sometimes called as a modulation current, to generate laser light, and those bias and modulation currents are varied depending on an ambient temperature of the LD to keep the optical output power and the ER in constant.
An average optical power Pave output from the LD depends on the bias current Ib and the modulation current Im applied to the LD. Generally, the bias current is set to be slightly greater than the threshold current (hereafter denoted as Ith) of the LD because Ib less than the Ith strongly affects the modulation characteristic of the LD, specifically, the rising of the optical output accompanies with some delays. The Im is adjusted, based on thus defined Ib, to show in the optical output therefrom the preset extinction ratio ER. FIG. 3 shows relations of the optical output power against the current supplied to the LD, where this relation is called as the I-L characteristic, in various temperatures.
As shown in FIG. 3, an LD inherently shows large temperature dependence in electrical and optical parameters thereof. The emission efficiency, which is sometimes called as the slope efficiency denoted as η, degrades and Ith increases in higher temperatures. Accordingly, it is necessary to set larger Ib and Im in high temperatures compared with those at a room temperature. On the other hand, the slope efficiency η becomes large while the threshold current Ith decreases in low temperatures, then, it is necessary to decrease both of Ib and Im in a low temperature compared with those at the room temperature.
An auto-power control (APC) has been well known to compensate the temperature dependence of the LD described above, where a portion of the light generated by an LD is monitored by a photodiode (PD), and each currents, Ib and Im, are adjusted to set the monitored output optical power the extinction ratio in constant. Three algorithms (1) to (3) below presented are applied to the APC:
(1) monitoring the peak power PHigh and the bottom power PLow independently, and controlling currents Ib and Im to keep the peak power PHigh and the bottom power PLow;
(2) monitoring the average power Pave and the extinction ratio ER of the optical output, and two currents Ib and Im are controlled to keep these parameters, Pave and ER; and
(3) controlling Ib to keep the monitored average power Pave in constant by the APC, and setting Im from thus determined Ib based on a look-up-table (LUT) that stores a relation between two current Ib and Im measured in advance to a practical operation.
Former two algorithms, (1) and (2), are necessary to monitor two parameters and to feedback the monitored results to two currents Ib and Im, which may enhance the preciseness and the stability of the APC control; but requires the PD to follow the modulation speed. The response of the PD may influence the stability and the preciseness of the APC control. Moreover, such a PD with superior frequency performance is generally expensive.
The third algorithm requires to prepare the LUT determining the relation between two currents Ib and Im. In order to make the size of the LUT compact, only the coarse data are prepared and conditions of currents, Ib and Im, are evaluated by interpolation/extrapolation of the coarse data. Thus, the third algorithm needs only one monitored condition; but the LUT is necessary to be prepared in advance to the practical operation. Moreover, when another condition for the extinction ratio ER is set, the LUT prepared above must be revised. In addition, when the APC determines the current Ib and the LUT determines the other current Im based on thus determined current Ib, newly evaluated current Im may vary the average power Pave, which revises not only the bias current Ib but the modulation current Im. Thus, the control loop to determine the currents, Ib and Im, occasionally becomes unstable.
Still further, when the LD is necessary to be operated in wider temperature range, for instance, between −40° C. to 85° C., and in higher speed, for instance, faster than 10 Gbps; electronic devices practically applicable thereto are so limited. One example shows that, even when the current conditions for an LD are satisfied in a high temperature by the third algorithm, the degradation of the electronic device, namely, an LD-driver, causes an extraordinary in the modulation current Im in a low temperature.